The present invention relates to a rear derailleur for a bicycle, intended for controlling the selective engagement of a bicycle chain on a series of coaxial pinions associated with the rear wheel of the bicycle. In particular, the invention relates to rear derailleurs of the type comprising:
a first articulating pin intended to be fixed to the bicycle frame with its axis substantially parallel to the axis of the pinions,
a first body mounted so that it is rotatable on the said first articulating pin,
first elastic means associated with the said first body tending to rotate the latter about the said first pin relative to the bicycle frame, with reference to the state when the derailleur is mounted on the frame, to an end-of-travel (stop) position,
a second body connected to the first body so as to be movable relative to the first body at least in a direction parallel to the axis of the pinions, the position of the said second body being controlled by means for controlling the derailleur,
a second articulating pin carried by the aforementioned second body and substantially parallel to the said first pin,
a rocking arm carrying rollers for transmission of the chain, rotatable on the said second pin, and
second elastic means interposed operatively between the aforementioned second body and the rocking arm, tending to rotate the latter about the said second pin to a stop position,
in such a way that, in the condition with the derailleur mounted on the bicycle, with the bicycle chain engaged with the rocking arm, the derailleur assumes an arrangement that depends on the equilibrium between the forces to which the derailleur is subjected, i.e. mainly the reaction of the chain on the rocking arm, the tension of the first elastic means and the tension of the second elastic means.
Derailleurs possessing the characteristics indicated above have been known and used for some time. A derailleur of this type is for example described and illustrated in French patent No. 1,258,146 (Juy), granted on Feb. 27, 1961. The same applicant has been manufacturing and marketing, at least since 1974, rear derailleurs for bicycles possessing the characteristics stated above, in which the first and second elastic means are two helical springs. In particular, in the derailleur that has been manufactured and marketed by the applicant since 1974 with the xe2x80x9cRallyxe2x80x9d trademark, the aforesaid first spring of the derailleur was interposed between the aforesaid first body and a stop washer, mounted on the aforesaid first articulating pin and provided with a tab capable of coming to rest against a stop-surface provided on the bicycle frame, in such a way that, with the derailleur mounted on the frame, the first spring was then interposed operatively between the first body of the derailleur and the bicycle frame.
U.S. Pat. No. 3,364,763 (Juy), published on Jan. 23, 1968, illustrates a derailleur of the type indicated above, in which the tension of the first spring can be adjusted by fitting one end of the spring in a hole selected between two holes provided on the first body.
U.S. Pat. No. 4,690,663 (Nagano), published on Jan. 9, 1987, also achieves adjustment of the tension of the first spring, but in this case rapidly (without requiring dismantling as in Juy) by adjusting the position of the stop-washer, which is provided with an adjusting screw that reacts against the bicycle frame, according to well-known technology (cf. U.S. Pat. No. 4,235,118).
All the solutions previously proposed, however, are not entirely satisfactory in as much as they involve limited angular ranges of adjustment and/or relatively long adjustment times. Moreover, the solution in U.S. Pat. No. 4,690,663 cannot always be used, as in some cases the bicycle frame is configured in a way that does not permit the use of the aforesaid adjusting screw. Finally, the use of the said screw is not completely reliable, since the screw itself operates independently in certain circumstances.
The aim of the present invention is to devise a rear derailleur for a bicycle of the type stated at the beginning, that is free from the aforementioned shortcomings and which, in particular, permits quick and simple adjustment of the configuration of the derailleur, with means that have an extremely simple structure that requires little space, and are such as to offer a relatively wide range of adjustment.
For achieving this aim, the object of the invention is a rear derailleur for a bicycle of the type indicated at the beginning of the present description, characterized in that means are provided for quick adjustment of only the aforesaid second elastic means, the said first elastic means being instead without means for quick adjustment.
The term xe2x80x9cmeans for quick adjustmentxe2x80x9d is intended to indicate any device capable of adjusting the elastic means without requiring the dismantling of parts, as is the case for example in solutions that envisage the possibility of fitting one end of a spring in any one hole of a series of holes provided in the respective connecting element.
According to the invention, the aforesaid means of adjustment are arranged to permit quick adjustment of the tension of the second elastic means interposed between the second body of the derailleur and the rocking arm. On changing the tension of the second elastic means, the system of forces to which the derailleur is subject is altered, so that the derailleur changes its configuration. In general this means that there is a change both in the angular position of the first body of the derailleur about the first axis of articulation relative to the bicycle frame, and in the angular position of the rocking arm.
In the preferred embodiment of the invention, the second elastic means consist of a helical spring, arranged around the said second pin and with one end connected rigidly to the said second body, the opposite end being connected rigidly to an adjusting device carried by the rocking arm, means being provided for adjustment of the angular position of the said adjusting device relative to the rocking arm about the axis of the second pin.
Preferably, the adjusting device comprises a disk with rotatable mounting on the rocking arm about the axis of the second pin and equipped with a toothed sector in engagement with a tangential adjusting worm gear that also has a rotatable mounting in the structure of the rocking arm. Owing to this arrangement, the adjusting device takes up much less space and does not involve substantial modifications relative to the structure of a conventional derailleur. Moreover, the use of a worm gear-toothed wheel connection makes it possible to obtain a relatively wide angular range of adjustment and to obtain adjustment over the whole range in a relatively short time.
The derailleur according to the invention envisages quick adjustment only of the second elastic means, i.e. it is without means for quick adjustment of the tension of the first elastic means, so as to avoid the shortcomings of the known technology described above.